Quite often integrated analog system components must be designed to operate in close conjunction with digital logic. Further, a typical desire is to fabricate related, if not mutually supportive, analog and digital system components on a common die. Close functional integration of analog and digital is naturally desirable for the ability to form high-level functional blocks. In any of these cases, the analog system components are typically required to operate from a common semiconductor, typically digital voltage supply level. This requirement is often a simple expedient arising from the cost effectiveness of using only a single, fixed potential power supply, such as a battery. Therefore, analog system components are often required to operate from power supply differences of between 1.5 and five volts, or more, over the age of a battery without loss of operational accuracy.
Typically, the accuracy of an analog system component will depend on the accuracy of its internal current and voltage reference circuits. Conventional current references utilize a Zener diode to accurately establish a reference voltage level. A reference current level can be derived from the reference voltage level through the conventional use of a simple current mirror amplifier circuit. However, the fabrication of an integrated Zener diode having a Zener threshold (V.sub.z) of less than about 6.2 volts is difficult and generally impractical particularly where other integrated devices are to be fabricated on the same substrate. To achieve Zener thresholds of incrementally less than 6.2 volts requires progressively higher doping densities in the fabrication of the diode. Such high doping densities are generally incompatible with the fabrication of other analog and digital components. Therefore, Zener diode based reference circuits are generally not used where the power supply potential difference is less than approximately 7 volts.
Band-gap reference circuits provide an alternate approach to obtaining reference current levels from low voltage supplies. Band-gap references generally rely on a difference between the semiconductor band-gaps of active semiconductor devices. However, band-gap references are quite complex to fabricate as compared to Zener references, are quite sensitive to fabrication process variations and require a well-behaved amplifier in the necessary reference level control feedback loop in order to maintain stable operation. Additionally, band-gap references typically require a relatively large integrated device surface area due to their circuit complexity.